This invention relates to an optical filter, and in particular to a narrow band filter.
Narrow band optical filters have been proposed comprising a polarisation diversity arrangement including a polarisation sensitive four port coupler on the input side, a polarisation transformer and a further similar coupler on the output side. Such couplers have the property, achieved by suitable choice of coupled length and guide spacing, that they exhibit "cross" transmission for TM polarisation (i.e. the signal crosses over to the other waveguide) and "bar" transmission for TE polarisation (i.e. the signal remains on the same waveguide) or vice versa. Thus if a source of mixed polarisation is applied to one port, TE and TM components will be separated at the output ports. If this is followed immediately by a second such coupler the components will recombine to appear at one output port only. However, if a polarisation converter or transformer is placed in between the two couplers, those components of the signals at the frequency of operation which are changed in polarisation will appear at the one output port of the second coupler.
A known polarisation converter comprises a pair of waveguides formed, for example, in a birefingent material such as X-cut Y-propagating Lithium Niobate. With no applied electric field, the TE and TM components are orthogonal to one another and propagate along the waveguide with different velocities. This gives rise to a characteristic beat wavelength where the two modes are in phase with one another. If an electric field is applied in the X-direction across the waveguide, the new "rotated" birefringence destroys the orthogonality of the two modes and each component splits into new "rotated" TE and TM components. By repeatedly applying the electric field at a spacing equal to the beat wavelength, the coupling between TE and TM components builds up in phase and one polarisation mode can be converted into the other. An electrode structure comprising a finite number of interdigitated electrodes spaced apart in the manner described above gives rise, in the frequency domain F to a sinc (sinF/F) response.
It has also been proposed to provide a ladder arrangement of spaced apart rungs, where each rung comprises a plurality of interdigitated electrode pairs spaced apart in the manner described above. This has the effect, in the frequency domain, of providing the device with a comb like response. A known way of tuning the filter is by applying a Z-directed E field--i.e. such that an electrode gap extends in parallel to the waveguide. This has the effect of changing the birefringence and thus the beat wavelength.
It is one object of the invention to increase the wavelength range over which a device of this general type may be operated.